Hard to put it in the right order, but basically a federacy of cities and non-populated areas.

National government consists of a unicameral legislature (non-parliamentarian), a courts system, and a small gendarmie. Each city gets a single representative at Senate. Not quite liberum veto but some high percentage of the body required to make something law. The legislature and courts are defined in individual charters, if something crazy happens new organizations can be made in the same manner. The only other thing the national government covers is a base civil/criminal code, and the administration of it in the non-populated areas and in cases spanning cities by the national courts. Judges appointed by legislature.

Cities can do whatever they want. The only important limits I can think of are not arbitrarilly depriving someone of life or freedom of movement.